The present invention relates to data error concealment in data processing systems. This invention is particularly advantageous for use in PCM (pulse code modulated) digital audio or audio/video reproducing systems.
A PCM technique, which is one of the signal processing techniques well known in the field of communication, has been recently used for recording/reproducing a video signal or audio signal. In particular, the PCM audio recording/reproducing technique, in which an audio signal is PCM recorded and then reproduced, has been rapidly developed to be put into practice as means for satisfying user's increased demands on the quality of sound. The development is based on the facts that the level of conventional analog recording techniques has been nearly saturated, the cost of A/D converters, D/A converters, IC memories or the like has been rapidly decreased, and code error correcting techniques have been improved.
In reproducing a PCM recorded audio signal, erroneous data, if any, is restored to correct data through a predetermined error correcting operation. But, the correction is impossible under some of various conditions of error occurrence. In that case, if erroneous data is left as it is, the data may appear as abnormal sounds. In order to avoid this, it is required to conceal the erroneous data.
Prior to explanation of error concealment, a brief description will now be made of PCM audio recording/reproducing. In a typical example of well known recording systems, an A/D converter converts analog audio signals of two channels L and R into PCM digital data signals of binary level "1" and "0" which contain 14 bits per unit data or word. Data blocks each composed of such six digitalized words are supplied to an error correcting word generator which uses six words in each data block to generate, through logical operations thereof, two 14-bit error correcting words P and Q for addition to that data block. For example, the first block is now constructed by an 8-word digital signal including L.sub.n (first data inputted from the L channel), R.sub.n (first data inputted from the R channel), L.sub.n+1, R.sub.n+1, R.sub.n+2, P.sub.n and Q.sub.n in this order. Thereafter, in order to provide against the occurrence of continuous data errors due to dropouts, etc. in data reproduction from a recording medium, the consecutive data is scrambled or interleaved in an interleave memory in a predetermined manner. For example, the first block is now composed of L.sub.n, R.sub.n-3D, L.sub.n+1-6D, R.sub.n+1-9D, L.sub.n+2-12D, R.sub.n+2-15D, P.sub.n-18D and Q.sub.n-21D (D=16H, H: block unit). The thus interleaved 8-word digital signal is supplied to an error detecting word generator where a 16-bit error data detecting word called a cyclic redundancy check code (CRCC) used for identifying an error word in each block is generated and added to that block. The CRCC added digital signals including nine words per unit block are recorded in a recording medium. (When a video tape is used, those signals are recorded after the conversion thereof into a standard TV signal format.) In a reproducing system, reverse processing is carried out. Digital signals produced from the recording medium are supplied to an error detecting circuit where the error detecting word CRCC is used to check for data error and a error flag is set if a data error is detected. The data is then restored to its original sequential order by a de-interleave memory. The de-interleaved data is supplied to an error correcting circuit. If any data error is detected in the de-interleaved data according to the error flag, the error is corrected through logical operations using the error correcting words P and Q so that the erroneous data is restored to correct data. For error exceeding the ability of correction using P and Q, the error is detected using CRCC and concealed. In some cases, the error correction using P and Q is not made but only the error concealment using CRCC is carried out.
One conventional approach for concealing data error is a so-called previous word hold method in which erroneous data is replaced or interpolated by the previous correct data, as shown in FIG. 1. Referring to FIG. 1, reference numeral 1 represents a correct waveform to be reproduced while numeral 2 represents a previous word held waveform. As apparent from the comparison between the waveform 1 including data, A, B, C, D, E and F and the waveform 2 including data A, B', C, E' and F, a considerable difference exists therebetween. Thus, the previous word hold method provides a problem in the quality of reproduced sounds.
Another conventional error concealment technique is an averaging method in which erroneous data is replaced or interpolated by a mean value between correct data immediately preceding and immediately following the erroneous data, as shown in FIG. 2. As apparent from the comparison between FIGS. 1 and 2, the averaging method provides a more merely excellent error concealment than the previous word hold method. Namely, the interpolated waveform 3 of FIG. 2 including data A, B", C, D, E" and F is nearer to the correct waveform 1 (including data A, B, C, D, E and F) than the waveform 2 of FIG. 1.
However, the actual use of the linear interpolation method will require different operations between times when an independent error is present in input data and when continuous errors are present in the input data. For example, for concealment of an independent error, three memories will be provided for storing three successive data words, since it is only necessary to know the correct data words immediately preceding and immediately following the erroneous data. For concealment of continuous errors, on the other hand, more than three memories will be required corresponding to the number of the continuous errors. In addition, a complicated control circuit will be required. But, in spite of the requirements of the increased number of memories and the complicated control circuit, which results in an overall circuitry of large size, it will be apparent that the effect of error concealment is small, since each of the continuous erroneous words is merely replaced by a mean value between correct words immediately preceding and immediately following the continuous erroneous words.